Such loudspeaker units conventionally developed includes a loudspeaker unit including a so-called track-shaped diaphragm 5, as shown in FIG. 13, having an aspect ratio of more than one, and consisting of a rectangular area 5a and a pair of semicircular areas 5b, 5b spreading on right and left of the rectangular area 5a (see Patent Document 1).
As shown in FIG. 12, the loudspeaker unit 4 includes a base frame 41 and a cover plate 42. The base frame 41 and the cover plate 42 are joined to each other at respective peripheries to form a flat casing 40.
Arranged in the casing 40 are the track-shaped diaphragm 5 and a drive mechanism 46 for driving the diaphragm 5.
The drive mechanism 46 includes a rectangular magnet 44 placed on the bottom face of the base frame 41, a rectangular toroidal coil 45 surrounding the magnet 44 and fixed to the back face of the diaphragm 5, and a rectangular toroidal metal yoke 43 surrounding the coil 45 and placed on the bottom face of the base frame 41. Application of a driving current to the coil 45 causes the diaphragm 5 to vibrate to produce sound.
As shown in FIG. 13 to FIG. 15, the diaphragm 5 includes a track-shaped thin plate made of a synthetic resin. The longitudinal ends of the diaphragm 5 are each in the form of a semicircle having a diameter equal to the length of the short side of the rectangular area 5a of the diaphragm 5.
The diaphragm 5 includes a track-shaped central portion 51 long in the longitudinal direction of the diaphragm 5, and a peripheral portion 52 with a track-shaped contour spreading around the central portion 51. The central portion 51 is formed with a certain or more thickness necessary to keep the strength of the diaphragm 5. The peripheral portion 52 is formed thinner than the central portion 51. The coil 45 is fixed within the central portion 51. The longitudinal ends of the central portion 51 are each in the form of a semicircle having a diameter approximately equal to the length of the short side of the coil 45.
A metal ring member 53 is joined to the peripheral portion 52 of the diaphragm 5 along the outer circumferential edge of the peripheral portion 52. The ring member 53 is pinched between the base frame 41 and the cover plate 42 as shown in FIG. 12.
In order to improve the loudspeaker characteristics, in particular, low-tone characteristics of the loudspeaker unit 4, it is necessary to decrease the area of the thicker formed central portion 51, and to increase the area of the peripheral portion 52, which greatly contributes to improvement in the low-tone characteristics.
A possible approach to decrease the area of the central portion 51 is to miniaturize the coil 45. However, with the configuration of the above loudspeaker unit 4, miniaturization of the coil 45 also brings the necessity of miniaturization of the magnet 44 placed inside the coil 45, which may cause a problem of low magnetic flux density passing through the coil 45, leading to low sound pressure.
Accordingly, in order to improve the low-tone characteristics, there has been proposed a loudspeaker unit 7, as shown in FIG. 16, having a magnet 22 disposed outside a coil 21.
The loudspeaker unit 7 includes a base frame 11 and a cover plate 12. The base frame 11 and the cover plate 12 are joined to each other at respective peripheries to form a flat casing 10.
Arranged in the casing 10 are a track-shaped diaphragm 6 and a drive mechanism 2 for driving the diaphragm 6.
The drive mechanism 2 includes a circular toroidal coil 21 fixed to the diaphragm 6, a circular toroidal magnet 22 surrounding the coil 21 and placed on the bottom face of the base frame 11, and a metal bottom plate 23 placed on the bottom face of the base frame 11. A cylindrical portion 24 projects from the bottom plate 23 toward the internal space of the coil 21.
As shown in FIG. 17 to FIG. 19, the diaphragm 6 includes a track-shaped thin plate made of a synthetic resin. The longitudinal ends of the diaphragm 6 are in the form of a semicircle having a diameter equal to the length of the short axis of the diaphragm 6.
The diaphragm 6 includes a track-shaped central portion 61 long in the longitudinal direction of the diaphragm 6, and a peripheral portion 62 spreading around the central portion 61. The central portion 61 is formed with a certain or more thickness necessary to keep the strength of the diaphragm 6. The peripheral portion 62 is formed thinner than the central portion 61. The coil 21 is fixed within the central portion 61. The longitudinal ends of the central portion 61 are in the form of a semicircle having a diameter approximately equal to the outer diameter of the coil 21.
A metal ring member 63 is joined to the peripheral portion 62 of the diaphragm 6 along the outer circumferential edge of the peripheral portion 62. The ring member 63 is pinched between the base frame 11 and the cover plate 12 as shown in FIG. 16.
The above loudspeaker unit 7 can provide improved low-tone characteristics because the coil 21 is miniaturized to decrease the area of the central portion 61 of the diaphragm 6 shown in FIG. 17 to the minimum that can secure a certain or more strength, and the area of the peripheral portion 62 is enlarged by the decreased area. Low sound pressure can be prevented by the disposition of the magnet 22 sized to secure a certain or more magnetic flux density, as shown in FIG. 16, in the great space created outside the coil 21 by the miniaturization of the coil 21.
Patent Document 1: JP-A-2003-37895 [H04R 9/04]